Typically, photon-counting detectors for detecting X-ray radiation are pixel-based X-ray detectors made from a direct-converting semiconductor material which are capable of counting and/or detecting individual photons of X-ray radiation that are incident on pixels of the X-ray radiation detector in an energy-resolving manner in that energy thresholds assigned to the pixels have been specified. When a photon of X-ray radiation strikes a pixel or penetrates the semiconductor material of the X-ray radiation detector, the photon interacts with the semiconductor material, whereupon free electrons are generated which, separated by an electric field, generate a charge pulse corresponding to the energy of the photon at electrodes of the pixel. The charge pulse is converted by way of signal-processing electronics comprising e.g. a preamplifier into a measurement voltage which is compared with threshold voltages representing different energy levels. In this way a specific energy can be assigned to a detected photon and the photon can be counted accordingly.
In order to be able to count photons of X-ray radiation striking such an X-ray radiation detector in an energy-resolved manner in the way described, the detector must first be calibrated energetically. The calibration is necessary for each measurement channel of the detector having a pixel and signal-processing electronics. In this case characteristic variables that describe the specific behavior of the detector material and of the signal-processing electronics are determined for each measurement channel. Setting the threshold voltages with maximum precision is critical in order to achieve a homogeneous response behavior of the detector and consequently e.g. to obtain optimally artifact-free and low-drift CT images when the detector is used in computed tomography applications.
During the calibration charge pulses of known magnitude corresponding to energies of photons of X-ray radiation are typically injected into the pixels and the response behavior of the measurement channel having the respective pixel is analyzed. From the analysis there results for each measurement channel a relation between injected pulses of a specific charge magnitude and the values of the measured voltage, such that threshold voltages can be specified in each case for the respective measurement channel for different charge pulse magnitudes or, as the case may be, energies of photons of X-ray radiation.
The calibration can be carried out electronically or using X-ray radiation. With electronic calibration the charge pulses are generated for example by way of clocked current sources or by way of the rapid charging and discharging of capacitances. For this purpose, however, the sources for generating the charge pulses must first be calibrated themselves. Furthermore, taking the specific behavior of the detector material into account during the charge collection is problematic in the case of electronic calibration.
There are various approaches with regard to calibration using X-ray radiation. According to a first approach the detector is calibrated based on the measurement or determination of the specific endpoint energy of the photons of different X-ray spectra. In practice, however, it proves difficult or complicated to detect the endpoint of the different X-ray spectra for the calibration in each case by way of the X-ray radiation detector.
A further possibility is to use radioactive sources that emit photons of X-ray radiation having a clearly defined energy. However, using radioactive sources is problematic in relation to the handling, the radiation shielding, of the available energies and of the flow of photons of X-ray radiation that can be generated, the latter because the sources become weaker with increasing age.
In theory a synchrotron could also be used for the calibration, though in practice this is ruled out on the grounds of the high degree of technical complexity and the only very limited availability.